The finite nature of oil resources and the ever-growing energy demand necessitate alternative energy conversion technology that is highly efficient and free of greenhouse gas emissions [1, 2]. Solid oxide fuel cells (SOFCs) utilizing oxide-ion conductors (e.g., Y-doped ZrO2 (YSZ)) due to higher efficiency (up to ˜80%), fuel flexibility, and combined heat power generation are being considered as alternative over conventional greenhouse gas emission system for stationary and mobile applications [3, 4]. However, high operating temperatures, typically between 800-1000° C., results in material degradation, coking in case of direct hydrocarbon fuels and sulfur poisoning in Ni-based cermet electrodes. The operating temperature of SOFCs is commonly dictated by the choice of electrolytes; hence, efforts have been focused on intermediate temperature (IT) (400-700° C.) ceramic proton conductors to reap many benefits especially with economic metal interconnects [5-8].
Among the known electrolytes, aliovalent-doped BaCeO3 (BCs) have demonstrated high proton conductivity (˜10−2 Scm−1 at 700° C.), but, their poor chemical stability to SOFC by-products such as H2O and CO2 has restricted them from being considered for proton conducting SOFCs [9, 10].
Persistent efforts to improve the key features of BCs have shown that doping with metal ions having larger ionic size compared to Ce increases proton conductivity, while doping with metal having higher electronegativity increases chemical stability. Yttrium (Y) remains one of the best candidates for doping for Ce in BCs, whereas ytterbium (Yb) and praseodymium (Pr) co-doping exhibited mixed ionic and electronic conduction [11, 12]. Comparison of ionic radii and electronegativity (see Table 1) suggests that both Y3+ and Gd3+ may be useful for Ce site doping in BCs. Additionally, computational studies using a ‘mean field approach’ where Gd3+ and Y3+ have showed the lowest solution energy for doping in the Ce site [13]. On the other hand, Sr-doping for Ba is proven to increase the phase stability under water vapor [14]. In contrast to BCs, BaZrO3-based proton conductors show appreciable chemical stability, but have poor sinter-ability and normally need very high temperature sintering (>1700° C.) that makes them unsuitable for electrode supported SOFCs [15-17].
US 20110084237 relates to membranes of proton-conducting ceramic said to be useful for conversion of hydrocarbon and steam into hydrogen comprising a certain porous support coated with a film of a perovskite-type oxide of the formula SrCe1−x−yZxMyO3−δ, where M is at least one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo, W, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb, x is 0 to about 0.15 and y is about 0.1 to about 0.3.
WO 2013/093044 relates inter alia to a perovskite type transition metal oxide which has formula(A1−xA′x)1−a(B1−yBy)1−bOd wherein:A and A′ are different from each other and A and A′ each independently comprises at least one element selected from the group consisting of strontium (Sr), yttrium (Y), samarium (Sm), cerium (Ce), bismuth (Bi), lanthanum (La), gadolinium (Gd), neodymium (Nd), praseodymium (Pr), calcium (Ca), barium (Ba), magnesium (Mg) and lead (Pb).;B and B′ are different from each other, and B and B′ each independently comprises at least one element selected from the group consisting of transition metal ions such as titanium (Ti), vanadium (V), manganese (Mn), cobalt (Co), iron (Fe), chromium (Cr), nickel (Ni) or copper (Cu); and gallium (Ga);x is between 0 and 1;y is between 0 and 1, anda, b and d correspond to site deviations from stoichiometry, which are reported to be useful in metal-air batteries.
Zuo et al. 2009 [26] relates to Ba(Zr0.1Ce0.7Y0.2)O3−δ as an electrolyte for low temperature solid oxide fuel cells. This reference is incorporated by reference herein in its entirety for descriptions of proton-conducting solid oxide fuel cells and assessments of electrolyte materials.